Gas turbines have difficulty competing with reciprocating internal combustion engines (i.e. gas and diesel engines) in applications where a large portion of the mission is spent at power levels significantly below maximum load, and where their inherent power density and emissions advantages are not substantially valued. This is due to the steep falloff in gas turbine thermal efficiency as power is reduced. This shortcoming has historically been addressed by the addition of a recuperator, which not only increases thermal efficiency at maximum load, but also reduces the efficiency lapse rate as load is reduced. In addition to the recuperator, variable power turbine inlet guide vanes are sometimes also included to further reduce the efficiency lapse.
Still even the recuperated gas turbine with variable power turbine inlet guide vanes realizes a severe thermal efficiency lapse when throttled from maximum load to low power settings. This is primarily the result of reductions in both cycle pressure ratio and turbine inlet temperature. The optimal throttling scheme typically consists of first shutting power turbine variable guide vane (VGV's), and then reducing turbine inlet temperature (TIT). Gas generator shaft speed is reduced during throttling, resulting in corresponding reductions in flow and pressure ratio (per the gas generator operating line). Reduction in turbomachinery efficiency as power is reduced further exacerbates the problem.